1. Field of the Invention
The present invention relates to the field of thin film deposition. More specifically, the present invention relates to improving uniform target erosion in a physical vapor deposition (PVD) hollow cathode magnetron sputter source.
2. The Background
The deposition of thin film layers is a common processing step in the fabrication of very large scale integrated (VLSI) circuits and ultra large scale integrated (ULSI) circuits on semiconductor substrates or wafers. A semiconductor wafer is the foundation from which is built a large quantity of discrete devices, commonly known in the art as integrated circuit chips. Metallic thin film layers are typically employed as device interconnects which are deposited on to a wafer by known physical or chemical vapor deposition techniques. In addition, it is also frequently required that small holes; referred to in the art as vias, or narrow grooves; referred to as trenches, be properly filled with metallization in order to provide electrical connection between device layers.
Recent innovations and cost constraints within the semiconductor industry have hastened the need to improve techniques used to dispose thin films on substrates. By example, wafer size has increased from 6-inch (150 mm) diameter to 8-inch (200 mm) diameter with a growing acceptance of even larger 12-inch (300 mm) diameter wafers. As wafer size increases, the ability to impart requisite directionality to thin films becomes increasingly difficult. Many current preferred methods of forming thin films, such as PVD sputtering, are only able to meet directionality and uniformity requirements by inducing a trade-off of slower processing rates. In today's highly competitive commercial semiconductor market improvements that carry with them increases in processing time are not viable economical alternatives. Similarly, the heightened complexity of current discrete devices have increased wafer densities and have led to vias and trenches with higher aspect ratios (depth of the via versus the width of the via) and smaller geometries. As the geometries of such vias and trenches decrease, it becomes increasingly more difficult to conformally deposit material throughout the entire depth of vias or grooves. Therefore, the need exists within the semiconductor industry to continue to strive for an adaptable, highly efficient means for thin film deposition.
Conventional PVD sputtering allows for the deposition of relatively pure thin films on substrates of various types and geometries. Standard sputtering is accomplished by creating at a relatively low pressure of a plasma forming gas a plasma comprising, typically, an inert gas, such as argon (Ar), in the vicinity of a target cathode which is made of the material to be deposited. Positively charged plasma atoms, known as ions, then strike the cathode target causing atoms of the target cathode to be ejected into the plasma. These target atoms then travel through the sputtering vacuum and are deposited onto the semiconductor substrate. Conventional diode PVD sputtering has shown to be both inefficient and, in some instances, incapable of providing required directionality to thin films when constructing VLSI and ULSI circuits. The plasma that is created with a standard PVD sputtering device lacks a sufficient amount of ionized target material atoms. The degree of ionization of a plasma is referred to in the art as the plasma intensity. The more intense the plasma, the greater the ability to steer and focus the plasma and, thus, impart an adequate amount of directionality to the ions in the plasma. By improving the ion directionality, it insures that the thin films being deposited have adequate coverage in vias and trenches. In addition, when the intensity of the plasma makes it more conducive to focusing operations overall processing times typically decrease and target material utilization is optimized.
As a means for overcoming the limitations of conventional PVD sputtering, use magnetic fields in magnetron sputtering devices have successfully been introduced into the process. These magnetron sputtering systems have seen wide-spread use in semiconductor manufacturing for the deposition of metallization layers, such as aluminum (Al), titanium (Ti), titanium nitride (TiN) and titanium tungsten (TiW) alloys. As with standard sputtering devices, the magnetron sputtering apparatus consists of a vacuum chamber which confines an inert support gas, commonly argon (Ar), at a relatively low pressure, typically 3-5 millitorr. An electrical field (E) is then created within the vacuum chamber by introducing a negative potential across the target cathode and creating an anode, typically, by means of grounding the overall sputter chamber or a using a self-biased floating anode. A magnetic field (B) is introduced into the vacuum chamber, typically in an orientation such that the field lines loop through the cathode for the purpose of creating and confining a plasma near the target cathode. As positive ions from the plasma strike the target cathode, atoms are ejected from the surface of the cathode. The magnetic field serves to attract an electron-rich portion of the plasma in the vicinity of the cathode. In addition, electrons trapped about the cathode allow for an increase in the collisions between the neutral atoms ejected from the surface of the target and the rapidly moving electrons. By increasing the quantity of collisions, the likelihood increases that a neutral ejected target atom will be struck by a sufficiently energetic particle within the plasma, thus causing the ejected target atom to lose one or more electrons and result in an ionized atom. By increasing the quantity of ionized target atoms within the plasma the overall effect is the increase in the plasma density, i.e., the number of particles in a given confined area. This increase in plasma density is also known in the art as an increase in the intensity of the plasma. As the plasma intensity increases so does the probability that further ionization of ejected target atoms will occur.
Magnetron sputtering devices have shown a wide variance of success in being able to deposit thin films efficiently and with the requisite step coverage and uniformity. A high percentage of such devices are limited in their ionization efficiency due in part to the fact that the vast majority of metal atoms ejected from the target remain neutral and the cathode configuration of such devices only result in a small volume of the plasma being retained in front of the target surface. Even with the use of magnetic fields to trap plasmas about the target cathode, the intensity of the plasma remains insufficient and, in certain embodiments, upwards of 98% or greater of the deposition material atoms remain un-ionized as they travel through the sputter chamber to the substrate. The general understanding is that atoms are ejected from the surface of the sputter target at random angles and that the mean-free path of travel between the target cathode and the substrate for these neutral metal atoms is reduced by random collisions with other target atoms or inert gas ions. When the predominately neutral atoms in these plasmas do come in contact with the substrate they characteristically do so over a wide range of angles, generally conforming to a cosine distribution. In particular, when atoms are disposed on substrate surfaces at angles less than normal it poses significant difficulty in uniformly filling trenches and interconnect vias. The emphasis on adequate step coverage of thin films is exasperated by the demands of the semiconductor industry. As the overall semiconductor geometries have shrunk and the chip densities have increased, so too have the demands on being able to impart required directionality to thin films in narrower and deeper vias and/or trenches.
The teachings found in U.S. Pat. No. 5,482,611 (the '611 or Helmer patent) entitled "Physical Vapor Deposition Employing Ion Extraction from a Plasma" have shown to be highly effective in providing a physical vapor deposition source which imparts an improved degree of ion directionality while achieving a commercially acceptable high deposition rate.
As shown in FIG. 1 the unique hollow cathode configuration of the magnetron disclosed in the '611 patent allows for a magnetic null 10 to exist on the radial axis 12 of the hollow cathode 14 a small distance from the cathode opening. This magnetic null 10 acts as a "cusp mirror" that reflects back into the hollow cathode cavity 16 most of the electrons susceptible to prematurely escaping from the plasma. The reflective nature of the cusp mirror allows for further electron interaction with ejected target material atoms and, thereby, increases the probability that target atom ionization will occur. In confining the plasma within the hollow cathode cavity 16 for a lengthened period of time, the '611 patent is successful in creating a high density plasma within the hollow region of the cathode. While most plasma densities prior to the inception of the '611 patent are of a maximum order of 10.sup.12 particles per cubic centimeter (particles/cc), this cathode configuration is able to achieve a much higher density plasma on the order of 10.sup.13 particles/cc or what amounts to an approximately 10 times increase in plasma density.
FIG. 2 depicts a cross sectional view of the discharge plasma and the plasma beam within the '611 hollow cathode magnetron sputtering system. Once the electrons in the discharge plasma leave the hollow cathode cavity 20, the cusp mirror 22 at the magnetic null region 24 isolates the electrons in the plasma beam 26 from the electrons in the hollow cathode cavity 20. The result is that the plasma beam 26 formed in the '611 patent is much more flexible than a standard plasma and can be manipulated and focused without affecting the discharge characteristics within the hollow cathode cavity 20. The cusp mirror 22 also embodies a loss-cone (not shown) which serves as the means whereby a plasma beam 26 is developed and emitted towards the substrate 28. The depiction of the loss cone region is purposely omitted from FIG. 3 because the loss cone concept is more closely related to the radial velocity along the z-axis as opposed to a spatial relationship within the cusp mirror. As previously discussed most electrons are reflected back into the hollow cathode cavity 20, however, a small percentage escape through the loss-cone towards the substrate 28. To maintain a neutral charge balance, ions are pulled along with the electrons by ambipolar diffusion creating the plasma beam 26. In order for electrons to escape through the loss-cone region of the cusp mirror 22, the magnetic moment of the escaped electrons must be smaller than the mirror ratio of the cusp mirror 22. As a result of this phenomena, the transverse velocity of the plasma beam 26 is greatly reduced. By limiting the beam velocity, the plasma beam 26 becomes much more conducive to steering, focusing or expanding the plasma beam 26 via the use of magnetic fields or electric fields.
The high level of plasma intensity that the '611 patent results in, is required to achieve the degree of ionization efficiency necessary to impart improved directionality and step coverage to the thin film and increase throughput to the overall deposition process. As shown in FIG. 3, when the substrate 30 is allowed to "float" electrically and a negative charge imparted from the plasma beam 32 is built up thereupon, target ions in the plasma beam 32, being positively charged, are attracted toward the surface of the substrate. As the plasma beam 32 approaches the substrate 30 it tends to hover above the surface of the substrate 30 by means of a thin low-voltage plasma sheath 34 which is created between the substrate 30 and the plasma beam 32. The potential gradient across this plasma sheath 34 is normal to the surface of the substrate 30 and, thus, taking into account the velocities and energies imparted to the ions, when the ions reach the substrate after being accelerated through the sheath, the angle of incidence approaches normal. When the plasma beam 32 is intense enough so that it contains an ample degree of ionized target atoms, directionality can be imparted to the plasma and ultimately to the thin film being deposited. The directionality problems exhibited when neutral target atoms predominate in the deposited film are diminished and proper step coverage of deep vias and narrow grooves can be realized.
In operation this configuration of a hollow cathode magnetron has shown to be highly effective in efficiently processing thin films with requisite step coverage, however; the overall efficiency of such a sputtering apparatus has been greatly impaired by non-uniform target erosion. When target erosion is limited to less than full-face erosion, the useful life of the target cathode is diminished and the likelihood of target particulate matter contaminating the deposited thin film becomes a concern. Shorter target life results in increased apparatus downtime associated with having to change targets frequently, and increases operating costs due to inefficient target utilization. Particulate matter, which flakes off from unevenly eroded targets, is especially a concern with target materials such as TiN and TiW.
FIG. 4 details the typical erosion profile that results during normal use of a prior art hollow cathode magnetron 40. In this instance, when the magnetic field is relatively uniform in strength between north magnetic poles 42 and south magnetic poles 44, the erosion groove 46 tends to have an asymmetric profile and forms around the cylindrical wall 48 near the cathode opening 50. FIG. 5 illustrates the rationale behind this erosion profile. An axial electric field exists in the hollow cathode 52 due in part to the incomplete shielding by the initially formed plasma, creating what is depicted here as constant electrical potential lines (E) 54. As the magnetic field lines (B) 56 become parallel to the constant electric potential lines (E) 54 near the interior cylindrical wall 58 of the hollow cathode 52 and close to the opening in the hollow cathode 52, the E.times.B, or magnetron action, would be at maximum strength. As a result, the erosion profile tends to center at this location 60 and its penetration into the target cathode is greatest at this locale. When such an erosion profile results, only approximately 30% of the target life is utilized prior to the target needing replacement.